1. Field of the Invention
The present invention relates generally to golf balls. More specifically, the present invention relates to methods for manufacturing golf balls and golf balls manufactured thereby.
2. Description of the Related Art
Golf balls may comprise one-piece constructions or they may include several layers including a core, one or more intermediate layers and an outer cover that surrounds any intermediate layer and the core. In multi-component golf balls, there exists an inner core. Often, this core is made by winding a band of elastomeric material about a spherical elastomeric or liquid-filled center. Alternatively, the core may be a unitary spherical core made of a suitable solid elastomeric material. One such material that is conventionally used for the core of golf balls is a base rubber, such as polybutadiene, which is cross-linked with a metal acrylate, such as zinc diacrylate.
In the construction of some multi-component golf balls, an intermediate boundary layer is provided outside and surrounding the core. This intermediate boundary layer is thus disposed between the core and the outer cover of the golf ball.
Located outwardly of the core and any intermediate boundary layer is a cover. The cover is typically made from any number of thermoplastic or thermosetting materials, including thermoplastic resins such as ionomeric, polyester, polyetherester or polyetheramide resins; thermoplastic or thermoset polyurethanes; natural or synthetic rubbers such as balata (natural or synthetic) or polybutadiene; or some combination of the above.
Golf balls are typically manufactured by various molding processes, whether one-component or multicomponent balls. Generally, the core of the golf ball is formed by casting, compression molding, injection molding or the like. If an intermediate boundary layer is desired, one or more intermediate boundary layers are added over the core by any number of molding operations, including casting, compression molding, and/or injection molding. The cover is then formed over the core and intermediate boundary layers, if present, through casting, compression molding, and/or injection molding.
One example of a conventional golf ball manufacturing process is described in U.S. Pat. No. 3,068,522, issued on Dec. 18, 1962 (xe2x80x9cthe ""522 patentxe2x80x9d). The manufacturing process disclosed in the ""522 patent uses a molding press having upper and lower die portions that surround a golf ball core. A retractable seat contacts the golf ball core to place the same in the desired orientation within the die cavity. The cover material is then injected into the die cavity to form the cover of the ball. The ""522 patent further discloses that the timing of the retraction of the retractable seat is critical and depends, in part, on the flow rate of the cover material into the mold cavity. In order to prevent the retardation of the flow rate or pre-hardening of the injected coating material, the ""522 patent teaches that in some instances, it may be necessary to heat the molding dies and pre-heat the golf ball core to a temperature slightly above ambient temperature.
Another example, U.S. Pat. No. 5,194,191 (xe2x80x9cthe ""191 patentxe2x80x9d), issued Mar. 16, 1993 discloses a method of preparing thread-wound golf balls. In the ""191 patent, a rubber thread or material is wound around a frozen core of material to form a core. Immediately thereafter, the core undergoes microwave heating to rapidly thaw the frozen core without heating the rubber thread layer and without moisture condensation taking place on the surface of the core. The method of the ""191 patent allegedly reduces the core thawing time and eliminates the need for a drying step.
These and other current golf ball manufacturing processes continue to suffer from a number of disadvantages. For example, when the outer cover of the golf ball is made from a thermoset material, e.g., thermoset polyurethane, the core and any intermediate boundary layer(s) tend to undergo thermal expansion during the casting of the outer cover, as heat is generated by the exothermic reaction of the thermoset processes used in the formation of the cover. As the cover forms, and before the cover develops sufficient green strength, the thermal expansion of the core and any intermediate boundary layer, in turn, may cause the outer cover of the golf ball to fracture or crack. Although this problem is particularly relevant to covers that are formed of thermoset polyurethane, it is not believed to be limited thereto. Similar problems may arise with other materials and processes.
Cracking is not the only problem with the current conventional methods of manufacturing golf balls. Since a temperature gradient exists between the core, intermediate layer, if any, and the outer cover, the central portion of the ball acts as a heat sink that absorbs heat given off during the cover-making process. In this regard, the reaction that takes place to cure the outer cover takes longer given the cooler temperature in the inner core of the ball. Production cycle times are thus adversely increased.
Consequently, there remains a need for methods of manufacturing golf balls that do not suffer from the above disadvantages. Moreover, the methods would preferably decrease the cycle time between production runs.
One aspect of the present invention is a method of manufacturing a golf ball. The method generally commences with heating a golf ball precursor product at a predetermined temperature and for a predetermined time period to achieve a predetermined volumetric thermal expansion of the golf ball precursor product. Next, a cover is applied over the thermally expanded golf ball precursor product.
The method may further include forming a golf ball precursor product. The forming of the golf ball precursor product may include forming a core and applying at least one boundary layer over the core. Alternatively, the forming of a golf ball precursor product may be limited to forming a core. The predetermined temperature may be a temperature within the range of about 135xc2x0 F. to about 175xc2x0 F., and more specifically a range of about 140xc2x0 F. to about 160xc2x0 F.
The at least one boundary layer may be a thermoplastic elastomer, and more specifically an ionomer. The cover may be a thermoset material, and more specifically a thermoset material with a resin from the group consisting of Allyl Pthalates like Diallyl Pthalates, (DAP) and Diallyl Iso Pthalates, (DIAP); Aminos; Cyanates; Epoxies; Phenolics; Unsaturated Polyesters; Bismaleimides; Polyurethanes; Silicones; Urethane Hybrids; Vinyl Esters; Liquid Nylon and Polydicyclopentadienes. The predetermined time period for heating may be about one hour at the predetermined temperature. The heating may be at a predetermined temperature and for a predetermined time period to achieve at least a 1.2% volumetric thermal expansion of the golf ball precursor product. Alternatively, heating the golf ball precursor product may be performed concurrently with forming the golf ball precursor product.
Another aspect of the present invention is a method of manufacturing a golf ball that includes forming a core, forming at least one boundary layer surrounding the core, and heating the core and boundary layer. The heating is conducted at a predetermined temperature and for a predetermined time period to achieve a predetermined volumetric thermal expansion of the core and at least one boundary layer. After heating, a cover is applied over the thermally expanded core and the at least one boundary layer.
Yet another aspect of the present invention is a golf ball having reduced susceptibility of cracking of a cover. The golf ball is produced in accordance with the method that includes heating a golf ball precursor product at a predetermined temperature and for a predetermined time period to achieve a predetermined volumetric thermal expansion of the golf ball precursor product. Next, a cover is applied over the thermally expanded golf ball precursor product.
It is the primary object of the present invention to provide a method for forming a golf ball that does not fracture during the cover forming process.
It is an additional object of the present invention to provide a method that preheats a core of a golf ball, and optional boundary layer, prior to forming a cover for the golf ball.
It is an additional object of the present invention to provide a golf ball that is formed by preheating the core of the golf ball, and optional boundary layer, prior to forming a cover for the golf ball.
Having briefly described the present invention, the above and further objects, features and advantages thereof will be recognized by those skilled in the pertinent art from the following detailed description of the invention when taken in conjunction with the accompanying drawings.